1. Field of the Invention
This invention generally relates to semiconductor manufacturing equipment and, more particularly, to the controlled delivery of source gas.
2. Description of Related Art
Advanced thin film materials are increasingly important in the manufacture of microelectronic devices. In contrast to traditional thin films, future thin films require new source materials that have low vapor pressures and that are often near their decomposition temperature when heated to achieve an appropriate vapor pressure. Some of the precursors, having both intrinsically low vapor pressure and low thermal decomposition temperature, are considered the best choices for deposition of films of tantalum oxide, tantalum nitride, titanium nitride, copper, and aluminum. For such applications, it is essential that the film morphology and composition be closely controllable. This in turn requires highly reliable and efficient means and methods for delivery of source reagents to the area of film formation.
In some cases, the delivery of reagents into the reactor in the vapor phase has proven difficult because of problems of premature decomposition or stoichiometry control. Examples include the deposition of tantalum oxide from the liquid precursor tantalum pentaethoxide (TAETO) and the deposition of titanium nitride from bis(dialkylamide)titanium reagents.
A precursor is the source of a vapor to be used in forming a thin film. Additionally, precursors often contain impurities, and the presence of those impurities can cause undesirable thermally activated chemical reactions at the vaporization zone, also resulting in formation of involatile solids and liquids at that location. For example, a variety of precursors, such as TAETO, are water-sensitive and hydrolysis can occur at the heated vaporizer zone forming tantalum oxide particulates that may be incorporated into the growing tantalum oxide film with deleterious effects.
Various source reagent delivery systems have been commonly employed to introduce vapors of source reagents to chemical vapor deposition (CVD) reactors. These include bubbler-based systems, liquid mass flow control systems, and liquid metering by pump systems. The use of CVD reactors is well known for thin film deposition and other thermal process steps required in the manufacture of integrated circuits.
FIG. 1A illustrates a typical bubbler-based delivery system, which includes an enclosed precursor chamber 10 at least partially submerged in the liquid of a heating bath 20. The temperature of the bath may be adjusted to heat or cool precursor chamber 10. In operation, precursor chamber 10 contains a precursor liquid. An inert carrier gas travels to precursor chamber 10 along a first pipe 30. The open end of first pipe 30 is located in the precursor liquid. The carrier gas exits the pipe and bubbles to the surface of the precursor liquid. Contained within precursor chamber 10 above the surface of the precursor liquid is a space 40. An input end for a second pipe 50 is located in space 40 above the surface of the precursor liquid. As the stream of the inert gas passes through the liquid precursor and bubbles to the liquid surface, precursor vapor attains its equilibrium vapor pressure more quickly. A xe2x80x9cspargerxe2x80x9d (a cap with multiple small perforations) is sometimes added to the end of first pipe 30 to ensure formation of small bubbles and rapid equilibration. The carrier gas and precursor vapor enter second pipe 50 and flow to a processing chamber, where the precursor vapor reacts upon a surface of a heated substrate. The temperature of pipe 50 is controlled by heating elements, such as heating coils 55, surrounding second pipe 50 to keep the precursor vapor from condensing during transport to the processing chamber.
The performance of bubbler-based delivery systems is complicated by the exponential dependence of liquid vapor pressure on temperature. Small changes in temperature can cause large changes in reagent delivery rate, leading to poor process control. Lower temperatures and/or higher flow rates of the bubbled carrier gas tend to lower vapor pressure. Therefore, fluctuations in carrier gas temperature and flow rate can cause the vapor pressure of the precursor liquid to fluctuate. Accordingly, the precursor vapor will not always be saturated, leading to fluctuating concentrations of the source reagent. Further, vapor concentration in the bubbler-based system is a function of carrier gas contact time in the fluid as the carrier gas bubbles to the surface. Thus, vapor concentration fluctuates over time as the level of precursor liquid in the precursor chamber changes with use.
FIG. 1B illustrates another common delivery system using a liquid mass flow controller (LMFC) to measure and control the flow rate of liquid precursor to a vaporizer. An enclosed precursor chamber 10 includes a precursor liquid. An inert gas travels to precursor chamber 10 along a first pipe 30. The open end of the pipe is located above the surface of the precursor liquid. Inert gas exits first pipe 30 and pressurizes the precursor liquid within precursor chamber 10. An input end for second pipe 50 is located in the precursor liquid. When the inert gas enters precursor chamber 10, the space above the precursor liquid becomes pressurized such that the level of the precursor liquid within precursor chamber 10 is lowered. Precursor liquid enters second pipe 50 and is transported to a LMFC 60. The precursor liquid exits LMFC 60 and is transported to a vaporizer 70. The precursor liquid is vaporized and is then typically entrained in a carrier gas which delivers it to the heated substrate. Gas exits the vaporizer through a heated pipe 90. The temperature of the pipe is controlled by heating elements, such as heating coils 95, surrounding the pipe.
Disadvantageously, liquid mass flow controllers present a number of drawbacks. LMFCs are extremely sensitive to particles and dissolved gases in the liquid precursor. LMFCs are also sensitive to variations in the temperature of the liquid precursor. Further, most LMFCs cannot operate at temperatures above 40xc2x0 C., a temperature below which some precursor liquids, such as TAETO, have high viscosity. Another drawback with LMFC-based systems can be attributed to the xe2x80x9cdead volumexe2x80x9d (e.g. piping) between the LFMC and the vaporizer. Any amount of liquid that remains in the dead volume can contribute in making an inaccurate delivery of source reagent to the vaporizer. These drawbacks can make volumetric control of the liquid precursor very difficult.
The aforementioned inaccuracies in volumetric control of the liquid precursor cause large inaccuracies in final delivery of the precursor vapor to the processing chamber since a small variation in liquid volume (flow rate) results in a large variation in gas volume (flow rate). Further, LMFC-based systems typically use a gas to assist in the vaporization of the liquid precursor, thereby increasing the probability of generating solid particles and aerosols. Additionally, spatial and temporal temperature variations usually occur in the vaporization zone, leading to inconsistent delivery of source reagents.
Finally, because of the temperature difference between the vaporizer and the pipe leading to the processor, such as pipe 90 in this example, condensation may occur during transport of the precursor vapor, which also contributes to inaccurate delivery of the source reagent.
FIG. 1C illustrates another well-known system using a pump for liquid metering of the precursor liquid to a vaporizer. Pump 80 pulls precursor liquid from precursor chamber 10 to a vaporizer 70. Vapor exits the vaporizer in a heated pipe 90. The temperature of the pipe is controlled by heating elements, such as heating coils 95 surrounding the pipe, to prevent condensation.
Pump-based systems have similar disadvantages as LMFC-based systems. Spatial and temporal temperature variations usually occur in the vaporization zone, leading to inconsistent delivery of source reagents. This system is also extremely sensitive to particles and gas dissolved in the liquid. Further, any dead volume of precursor liquid delivered to the vaporizer will increase inaccurate delivery of source gas. Thus, delivery of precursor vapor to the processing chamber lacks high accuracy in this system since a small error in liquid volume measurement or control leads to a large error in vapor volume. Finally, most pumps cannot tolerate high temperatures (maximum 50xc2x0 C.), below which some precursor liquids have high viscosity.
Therefore, what is needed is a precursor vapor delivery method and system with control for precise stoichiometry by limiting fluctuations in the concentration of source gas delivered to the processing chamber.
In accordance with the present invention, a method and a system are provided for the controlled delivery of source gas to a processing chamber. A source gas delivery method includes providing a precursor chamber configured to hold precursor vapor, providing saturated precursor vapor at a selected pressure within the precursor chamber, and flowing saturated precursor vapor from the precursor chamber to a processing chamber until a selected pressure is provided within the processing chamber. Advantageously, the present invention provides and controls precise stoichiometry involved in the process reactions by delivering accurate amounts of precursor vapor to the processing chamber.
In another aspect of the present invention, a source gas delivery method includes providing a precursor chamber configured to hold precursor vapor, providing saturated precursor vapor at a selected pressure within the precursor chamber, and diffusing saturated precursor vapor from the precursor chamber to a processing chamber until a selected pressure is provided within the processing chamber.
In yet another aspect of the present invention, a source gas delivery system includes a precursor chamber configured to hold precursor vapor, a heat source for heating a precursor liquid to provide saturated precursor vapor at a selected pressure within the precursor chamber, and a vapor pathway allowing saturated precursor vapor to enter a processing chamber until a selected pressure is provided in the processing chamber.
In FIG. 4, an advantage of the present invention is illustrated using graph 400, which shows how the concentration of the vapor delivered to a processing chamber can vary over time. In a typical vapor delivery system, precursor vapor is delivered at a fairly constant level 402, with fluctuations in the concentration occurring constantly. This manner of precursor vapor delivery ensures that enough source gas is available during processing operations. Unfortunately, most of the source gas is wasted since only a small percentage is consumed in the processing operation. The wasted gas must be vented, which can also require special procedures and additional treatments.
In the present invention, once a predetermined concentration of vapor 404 is delivered to the process chamber, the delivery is stopped. As the vapor reagents are consumed in the processing, the concentration level 404 drops, but the amount of wasted gas is substantially reduced since the difference between concentration level 402 and concentration level 404 will not have to be purged.
These and other features and advantages of the present invention will be more readily apparent from the detailed description of the embodiments set forth below taken in conjunction with the accompanying drawings.